Double down if the dealer is showing a 3, 4, 5 or 6.
Stand if the dealer has a 2, 7, or 8.
Take a hit if the dealer has a 9, 10, or ace.
The reason for doubling down against the 3 -6 is simple - you have a combination of a possible dealer bust combined with a potentially improved hand, with no chance of getting dealt a bust yourself. This situation is so good for the player that you should take advantage of it by getting more money in action.
You always put the dealer's hole card on a ten, so if she has a 2 or 7 or 8, you assume she has a 12, 17, or 18. If any of those are true, then hitting the soft 18 has a good chance of hurting your hand instead of helping. And a dealer total of 12 is a lot less likely to bust then a 13 -16. And a dealer will stand on the 17 or 18.
If the dealer has a 9 through an ace, you're hoping to improve your hand and win against her potential big hand.